1. Field of the Invention
The invention relates to a method of fabricating a thin-film semiconductor device and an apparatus for fabricating the same, and more particularly to a method of forming a thin silicon film used for a crystalline silicon thin-film transistor, and an interface between semiconductor and an insulating film, used for a field effect transistor, and further to an apparatus of fabricating such a thin silicon film and such an interface.
2. Description of the Related Art
Japanese Patent Publication No. 7-118443 has suggested a method including the step of radiating laser beam having a short wavelength, to an amorphous silicon thin film formed on an amorphous substrate, to thereby fabricate a thin film transistor. The method makes it possible to crystallize amorphous silicon without wholly heating the substrate, and hence, makes it possible to fabricate a semiconductor device or a semiconductor integrated circuit on a substrate having a wide area such as a substrate to be used for a liquid crystal display, or a cheap substrate such as glass.
Japanese Unexamined Patent Publication No. 9-320961 has suggested a method of fabricating a thin-film transistor. In this method, the steps of forming an amorphous silicon thin film to which laser beam is to be radiated, radiating laser beam to the amorphous silicon thin film, carrying out hydrogenation in plasma, and forming a gate insulating film are carried out in this sequence or other sequences without exposure to atmosphere.
The Publication also discloses an apparatus of fabricating a semiconductor thin film, including a first chamber in which a substrate is loaded in vacuum, a second chamber in which silicon is formed, a third chamber in which laser beam is radiated, a fourth chamber in which an insulating film is formed, a fifth chamber in which annealing is carried out in hydrogen atmosphere, a sixth chamber in which a substrate is unloaded, and a seventh chamber through which a substrate is transferred to other chambers.
A glass substrate is transferred into the apparatus from the first chamber. The glass substrate can be transferred to any one of the chambers through the seventh chamber and a vacuum valve. The first to seventh chambers are equipped with a gas exhaust system independently of one another, and hence, can exhaust reactive gas, inert gas and other gases introduced from gas introducers in the steps of forming a silicon film, forming an insulating film, and annealing.
When a substrate is transferred out of any one of the chambers, the chamber is sufficiently exhausted. When the chamber has almost the same pressure as that of the seventh chamber, a vacuum valve is released, and then, a substrate is taken out of the chamber by means of a robot. Then, the vacuum valve is closed.
The substrate is introduced into a next chamber having almost the same vacuum degree as that of the seventh chamber after releasing a vacuum valve, transferring the substrate into the third chamber, and closing the vacuum valve. After closing the vacuum valve, a process gas is introduced into the chamber, and a pressure and a temperature in the chamber are adjusted to a predetermined pressure and temperature. Then, laser beam is radiated to the substrate.
The substrate is transferred among the first to sixth chambers in such a manner as mentioned above. A plurality of substrates can be transferred among the chambers through the use of a plurality of robots, in which case, the chambers are sufficiently exhausted. A vacuum valve may be released and closed, and the substrate may be transferred after the chambers are caused to have almost the same pressure in inert gas, nitrogen gas or hydrogen gas atmosphere.
The substrate is transferred between the first chamber and atmosphere and further between the sixth chamber and atmosphere, after nitrogen or inert gas is leaked with the vacuum valve being closed, and a valve is released to thereby allow the first and sixth chambers to be in fluid communication with atmosphere.
Thus, all the steps are carried out without exposure to atmosphere. The reason is as follows. Since a surface of silicon formed by laser crystallization is quite active, contaminants are likely to be adhered to the surface, if the silicon is exposed to atmosphere. This results in degradation in performances of resultant TFT. As an alternative, there is dispersion among performances of TFT. In order to avoid such degradation or dispersion, all the steps are carried out without exposure to atmosphere.
The inventor conducted the experiment in which excimer laser crystallization and formation of a silicon dioxide film were carried out in the same apparatus in two cases in one of which a substrate was exposed to atmosphere and in the other of which a substrate was not exposed to atmosphere. Herein, xe2x80x9cthe same apparatusxe2x80x9d includes a case in which a substrate is transferred to another apparatus without exposure to atmosphere. In the case in which a substrate was not exposed to atmosphere, a fabrication yield was much enhanced because dusts and particles were prevented from adhering to a product. However, it was also found out that such enhancement in a fabrication yield can be obtained by enhancing cleanness in a clean room. A fabrication yield was enhanced best in an film-forming apparatus including a washing device therein.
Comparing a trap level density in a silicon film to an interface level density (or density of electric charge in a fixed oxide film), the trap level density is obviously greater than the interface level density. That is, there is a problem of insufficiency in performances of a silicon film (or a trap level density) in order to have sufficient cleanness in a product having a silicon film and a gate insulating film both formed without exposure to atmosphere in the same apparatus.
The inventor analyzed the above-mentioned problem, and resultingly, found out the following problems in connection with steps of fabricating a silicon film and a gate insulating film and an apparatus of fabricating the same.
The first problem is as follows. For instance, in a cluster tool type apparatus suggested in Japanese Unexamined Patent Publication No. 7-99321, a plurality of chambers is arranged having its own purpose. Hence, it is quite difficult to keep a chamber located at a core, away from contaminants. There occurs cross-contamination in transfer of a substrate between chambers, even though the cross-contamination is slight.
The second problem is as follows. For instance, an in-line type apparatus suggested in Japanese Unexamined Patent Publication No. 5-182923 is accompanied with a problem that it is unavoidable generation of minute dust, in particular, metal particles due to a great frictional area between parts in vacuum.
The third problem is as follows. Silicon crystallized by laser beam would have a quite active surface. For instance, if silicon is coated with active species having energy, for instance, radical species such as hydrogen radical, oxygen radical, hydrogen ion, oxygen ion, ion species or ozone, after the silicon has been crystallized by laser beam, but before a gate insulating film is formed, contaminants adhered to a wall of a chamber and metal constituting a wall of a chamber are excited, resulting in that atmosphere in which a substrate is put is contaminated.
The fourth problem is that since laser radiation in oxidation atmosphere reflects dispersion in laser intensity in a step of introducing oxygen into silicon, there would be dispersion in a concentration of oxygen in a silicon film. This results in non-uniformity in characteristics of resultant silicon films.
The fifth problem is as follows. When a plurality of steps are to be successively carried out without exposure to atmosphere, for instance, steps of crystallizing a silicon film by means of laser beam and thereafter forming a gate insulating film, though it is possible to reduce contaminants adhered to the silicon film by not exposing the silicon film to atmosphere, the above-mentioned problems still interfere with fabrication of a semiconductor device. A conventional method of fabricating a semiconductor device such as LSI includes a step of carrying out thermal oxidation at about 1000 degrees centigrade to form an interface in a crystalline silicon film. This means that it is necessary to control contaminants even in a vacuum apparatus.
There has been suggested remote plasma chemical vapor deposition (CVD) to reduce damage caused by plasma and form a qualified gate insulating film. For instance, Japanese Unexamined Patent Publication No. 5-21393 has suggested a plasma CVD apparatus including a first chamber in which plasma is generated and a second chamber in which a substrate is processed. The first chamber is separate from the second chamber. It is considered that the suggested apparatus would accomplish a low density of electric charge of a fixed oxide film in the range of 1xc3x971011 to 1xc3x971012cmxe2x88x922, and a low interface level density smaller than 6xc3x971010cmxe2x88x922 eVxe2x88x922. However, this advantage of accomplishing such low densities is restricted to performances of a silicon film.
The sixth problem is as follows. A chamber in which a substrate is processed is frequently caused to have a high vacuum degree or a low pressure in order to prevent contaminants from adhering to a surface of a substrate. In particular, in plasma CVD where a film is to be formed at a pressure smaller than atmospheric pressure, a chamber is exhausted almost to an ultimate vacuum degree except while a film is being formed. For the same reason, a chamber through which a substrate is transferred to another chambers, in a batch-type apparatus, is exhausted almost to and kept at an ultimate vacuum degree.
In an excimer laser radiation apparatus, excimer laser beam is often radiated in vacuum atmosphere. However, silicon particles separated from a silicon film during laser radiation are adhered to a window through which laser beam is introduced into the apparatus, resulting in reduction in a laser transmission rate with the lapse of time.
In order to solve this problem, Japanese Unexamined Patent Publication No. 9-139356 has suggested a laser annealing apparatus in which excimer laser beam is focused on a window to which silicon is adhered, to thereby thermally decompose silicon. However, in this apparatus, since a laser beam which is usually focused on a surface of a substrate is focused on a surface of the window having a different optical length, it would be absolutely necessary to rearrange optical systems. When there are employed optical systems having a small focal length, in particular, when mask projection method is carried out, it is necessary to accurately position optical systems, resulting in reduction in an operation efficiency of the apparatus.
Japanese Unexamined Patent Publication No. 3-292719 has suggested a method of forming a silicon semiconductor layer, including the steps of forming a silicon semiconductor layer on an insulating substrate at a temperature equal to or lower than 600 degrees centigrade, and radiating energy beam to the silicon semiconductor layer to thereby turn the silicon semiconductor layer into polysilicon.
Japanese Unexamined Patent Publication No. 9-36376 has suggested a method of fabricating a thin-film semiconductor device, including the steps of forming a thin semiconductor film on an insulating substrate, the thin semiconductor film containing hydrogen at 10% or greater as well as inert impurities, annealing the insulating substrate at 350 degrees centigrade or higher to thereby remove hydrogen such that the thin semiconductor film contains hydrogen at 10% or smaller, and radiating laser beam to the thin semiconductor film to thereby activate the impurities.
Japanese Unexamined Patent Publication No. 5-326397 has suggested a method of fabricating a semiconductor device, including the steps of forming an amorphous semiconductor film on a semiconductor substrate at such a temperature that the amorphous semiconductor film has a planar surface at a pressure smaller than an atmospheric pressure, annealing the semiconductor substrate in inert gas atmosphere at a temperature higher than a temperature at which the amorphous semiconductor film is formed, to thereby turn the amorphous semiconductor film into a polysilicon film having irregularities at a surface thereof, the polysilicon film acting as a first electrode of a capacitor, forming a dielectric film on the first electrode, and forming a second electrode on the dielectric film.
Japanese Unexamined Patent Publication No. 5-182919 has suggested a method of fabricating a thin polysilicon film, including the steps of forming an amorphous silicon film on a glass substrate by LPCVD, putting the substrate in an oxygen atmosphere to thereby oxidize a surface of the amorphous silicon film, and annealing the amorphous silicon film in inert gas atmosphere at 600 degrees centigrade or smaller to thereby turn the amorphous silicon film into polysilicon.
Japanese Unexamined Patent Publication No. 11-17185 has suggested a method of fabricating a liquid crystal display device, including the steps of forming a semiconductor film almost all over a substrate to define an insulating gate type transistor, heating and recrystallizing the semiconductor film, forming a gate insulating film of the insulating gate type transistor on the thus recrysallized semiconductor film, and forming a gate electrode of the insulating gate type transistor almost all over the gate insulating film. All of the steps are carried out in an apparatus which is kept vacuous.
Japanese Unexamined Patent Publication No. 10-149984 has suggested a method of forming polysilicon, including the steps of radiating laser beam to an amorphous silicon film formed on a substrate, in a hermetically sealed chamber, and annealing the amorphous silicon film to thereby turn the film into polysilicon. The chamber is designed to have a vacuum degree of 0.1 Torr or higher, and have atmosphere of at least one of hydrogen, nitrogen and inert gas.
Japanese Unexamined Patent Publication No. 10-116989 has suggested a method of fabricating a thin-film transistor, including the steps of forming a semiconductor film on a substrate without exposure to atmosphere, crystallizing the semiconductor film in non-oxidizing atmosphere without exposure of the substrate to atmosphere, forming a first gate insulating film on the semiconductor film without exposure of the substrate to atmosphere, annealing the first gate insulating film and the semiconductor film, patterning the first gate insulating film and the semiconductor film, hydrogenating the substrate, and forming a second gate insulating film on the first gate insulating film.
Japanese Unexamined Patent Publication No. 9-17729 has suggested a method of fabricating a semiconductor device, including the steps of forming a first insulating film, a thin amorphous semiconductor film, and a second insulating film on an upper surface of an insulating substrate without exposure to atmosphere, and radiating laser beam through a lower surface of the insulating substrate to thereby turn the thin amorphous semiconductor film into crystal.
Japanese Unexamined Patent Publication No. 9-7911 has suggested an apparatus for fabricating a semiconductor device, including (a) a laser annealing unit having a chamber in which a substrate is kept hermetically sealed, and in which laser beam is radiated to the substrate, (b) a film-forming unit having a chamber in which a substrate is kept hermetically sealed, and in which a thin film is formed on the substrate, and (c) a transfer unit which transfers the substrate between the chambers with the substrate being kept hermetically sealed.
In view of the above-mentioned problems, it is an object of the present invention to provide a method and an apparatus both of which is capable of fabricating a thin silicon film having a low trap level density, by radiating laser beam thereto.
It is also an object of the present invention to provide a method and an apparatus both of which are capable of providing an interface between semiconductor and an insulating film which interface has a small interface level density, that is, a field effect transistor having improved characteristics.
Another object of the present invention is to provide a method and an apparatus both of which are capable of forming the above-mentioned thin silicon film at a temperature in the range of room temperature to 600 degrees centigrade for the purpose of allowing to use a cheap glass substrate.
In one aspect of the present invention, there is provided a method of fabricating a thin-film semiconductor device, comprising the steps of (a) melting and recrystallizing at least a surface of a thin semiconductor film formed on a substrate, in a pressure lower than an atmospheric pressure or in inert gas atmosphere, (b) keeping the substrate in atmosphere including oxygen gas, and (c) forming an insulating film on the thin semiconductor film with the substrate being kept in a pressure lower than an atmospheric pressure or inert gas atmosphere.
It is preferable that the atmosphere predominantly includes oxygen gas or the atmosphere includes only oxygen gas.
It is know that if silicon is left in atmosphere, there would be formed a natural oxidation film on an active surface of silicon. Since organic substances and/or metal particles floating in the air are absorbed into the natural oxidation film, the natural oxidation film formed in atmosphere is not suitable for formation of a clean interface, which is an object of the present invention.
In a conventional method of fabricating a bipolar transistor, in order to form silicon crystal by epitaxial growth, the following steps are carried out: removing a natural oxidation film through the use of hydrofluoric acid, forming a chemical oxide film through the use of heated solution of ammonia/H2O2/H2O or HCl/H2O2/H2O, annealing at 1000 degrees centigrade or greater in an epitaxial growth furnace through the use of hydrogen gas, to thereby remove the chemical oxide film and resultingly form a clean surface of silicon, and growing a film.
However, when a step which is to be carried out at a temperature in the range of room temperature to 600 degrees centigrade has to be carried out, the high-temperature step in the above-mentioned conventional method can not be selected.
In addition, since a surface of a thin silicon film having been crystallized by laser beams has experienced a temperature of 1000 degrees centigrade or higher at which silicon would be molten, even in an order of nanoseconds, the surface is in quite active condition. Hence, even in a vacuous chamber, contaminants would be readily adhered to the surface, if atmosphere in the chamber is suitably controlled.
In contrast, in accordance with the present invention, highly purified oxygen gas is introduced into a chamber just after silicon has been crystallized by laser beams, to thereby form a natural oxidation film having a low concentration of contaminants. By forming such a natural oxidation film on a surface of silicon, it would be possible to prevent contaminants from being adhered to a surface of silicon in various chambers such as a chamber in which laser beam is radiated, a chamber through which a substrate is transferred to another chamber, or a chamber in which a film is formed.
If active gases such as radicals or ions are used for formation of a natural oxidation film, it would be possible to effectively form a natural oxidation film and establish hydrogen passivation. However, the use of those active gases might cause absorption of contaminants adhered to a wall of a chamber into a natural oxidation film, and thus, it is not preferable to use such active gases.
The inventor conducted the experience in which silicon dioxide films were formed on a silicon wafer through the use of oxygen gases A, B and C having different purities, and then, leakage current was measured for each one of oxygen gases A, B and C.
There were obtained current densities X1, X2 and X3, when an electric field of 5 MV/cm was applied to the samples in which the gases A, B and C were used, respectively. The relation among X1, X2 and X3 is X1 greater than X2 greater than X3.
Since carbon existing in a silicon dioxide film would cause current leakage, it is necessary to reduce a concentration of carbon. In addition, since metals such as Na, K or Li exist as movable ions in an oxide film, they would cause a threshold value to shift. Accordingly, gas having a high purity, such as O2, N2O, silane or disilane is necessary to be prepared for formation of a thin silicon film, oxidation of a surface of silicon or deposition of a silicon dioxide film.
Oxygen gas is fractionated from air through low temperature processing. In fractionation, hydrocarbon such as methane is all residual in oxygen gas because a boiling point of methane is higher than a boiling point of oxygen. Specifically, a boiling point of oxygen is xe2x88x92183 degrees centigrade, a boiling point of methane is xe2x88x92162 degrees centigrade, and a boiling point of nitrogen is xe2x88x92196 degrees centigrade. As a result, hydrocarbon in the air is condensed and residual in oxygen gas.
The following method is carried out in order to remove such hydrocarbon. First, porous catalyst such as Pt or Pd is heated. Then, hydrocarbon is made to react with oxygen to thereby form CO2 and H2O. Those CO2 and H2O are absorbed into an absorber. As a result, it is possible to reduce a concentration of hydrocarbon in oxygen to 0.1 ppm or smaller, and concentrations of CO2 and CO to 0.1 ppm or smaller. Hence, an apparatus for refining oxygen may be arranged upstream of a gas supplier which supplies gases to a process apparatus.
It is known that argon and nitrogen are residual in a step of fractionation. However, highly purified argon or highly purified nitrogen in an order of ppm does not cause any problems in the present invention. For instance, even if hydrogen, nitrogen or inert gas such as argon each having a purity of 99.9999% or higher is mixed with oxygen gas, such mixture gas does not cause nay problems.
As a result of the experiments the inventor conducted, the following was found out.
It is preferable that the atmosphere includes oxygen gas having purity of 99.999% or greater.
It is preferable that the atmosphere further includes hydrogen gas. having purity of 99.999% or greater.
It is preferable that the atmosphere further includes nitrogen gas having purity of 99.999% or greater.
It is preferable that the atmosphere further includes inert gas having purity of 99.999% or greater.
It is preferable that a process gas used for forming the insulating film has purity of 99.999% or greater.
It is preferable that a process gas used in the method includes hydrocarbon (CnHm) species having a total concentration of 1 ppm or smaller.
It is preferable that recrystallization in the step (a) is carried out through laser radiation.
There is further provided a method of fabricating a thin-film semiconductor device, including the steps, in sequence, of (a) introducing a substrate into a vacuous chamber, (b) introducing non-reactive gas into the chamber, (c) radiating laser beam to the substrate in the chamber, (d) introducing oxygen gas into the chamber, (e) exhausting the non-reactive gas and the oxygen gas until a pressure in the chamber is reduced down to a predetermined pressure, and (f) taking the substrate out of the chamber.
It is preferable that the non-reactive gas is selected from the group consisting of nitrogen gas, inert gas and hydrogen gas alone or in combination.
It is preferable that the method further includes the step (g) of radiating laser beam to a window through which laser beam is radiated into the chamber such that laser beam is not radiated to a completed region of the substrate, the step (g) being to be carried out between the steps (d) and (e).
It is preferable that the non-reactive gas is kept introduced into the chamber to cause the non-reactive gas to have a constant pressure.
It is preferable that the method further includes the step of heating the substrate.
There is still further provided a method of fabricating a thin-film semiconductor device, including the steps, in sequence, of (a) introducing a substrate into a vacuous chamber, (b) introducing non-reactive gas into the chamber, (c) radiating laser beam to the substrate in the chamber, (d) stopping introduction of the non-reactive gas into the chamber, (e) introducing oxygen gas into the chamber, (f) transferring the substrate from the chamber into a second chamber having the same internal pressure as that of the chamber.
It is preferable that the method further includes the steps of (g) radiating laser beam to a window through which laser beam is radiated into the chamber such that laser beam is not radiated to a completed region of the substrate, after the substrate has been transferred into the second chamber, and (h) exhausting the non-reactive gas and the oxygen gas until a pressure in the chamber is reduced down to a predetermined pressure.
In another aspect of the present invention, there is provided an apparatus for fabricating a thin-film semiconductor device, including (a) a first chamber which is capable of keeping the first chamber in various pressure atmospheres, (b) an energy beam radiator which radiates energy beam to at least a surface of a semiconductor thin film formed on a substrate, (c) a carrier which carries the substrate between the first chamber and a second chamber which is capable of accomplishing the same pressure atmosphere as that of the first chamber, (d) a first gas-introducer which introduces nitrogen gas or inert gas into the first chamber, (e) a gas pressure regulator which keeps the first and second chambers in a predetermined pressure atmosphere, (f) a second gas-introducer which introduces oxygen gas into the first chamber, and (g) a controller which controls operation of the energy beam radiator, the carrier, the first gas-introducer, the gas pressure regulator, and the second gas-introducer.
It is preferable that the controller controls operation of the energy beam radiator, the carrier, the first gas-introducer, the gas pressure regulator, and the second gas-introducer such that the following steps are carried out in this sequence: (a) adjusting pressures in the first and second chambers so that the pressures are almost equal to each other, (b) introducing the substrate into the first chamber from the second chamber, (c) introducing the nitrogen gas or inert gas into the first chamber, (d) radiating laser beam to the semiconductor thin film, and (e) introducing oxygen gas into the first chamber.
It is preferable that the apparatus further includes a heater for heating the substrate.
It is preferable that the controller controls the laser beam radiator such that laser beam is radiated to the first chamber without radiating laser beam to a completed region of the substrate, after the oxygen gas has been introduced into the first chamber.
In order to successively carry out a step in which laser is radiated at a pressure almost equal to atmospheric pressure, a step of carrying out CVD at a vacuum degree of a couple of Torrs, and a step of transferring a substrate between those steps without exposure of the substrate to atmosphere, it is preferable to reduce a difference in a pressure between chambers between which the substrate is transferred.
After laser beam has been radiated in nitrogen or inert gas atmosphere, but before nitrogen or inert gas is compulsively exhausted, oxygen gas is introduced into a chamber in which a laser beam is radiated, to thereby oxidize an active surface of silicon. At this time, silicon adhered to a window through which laser beam is introduced into the chamber is also oxidized, because silicon has an active surface. With oxygen being introduced into the chamber, the laser beam is introduced into the chamber such that the laser beam is not radiated to effective areas.
In particular, when ultra-violet ray is used as a laser beam, oxygen gas is decomposed by the ultra-violet ray, and at the same time, silicon having been adhered to the window and having not been oxidized by oxygen gas is heated. The thus decomposed, active oxygen reacts with the thus heated silicon to thereby form silicon dioxide, ensuring that reduction in a transmission rate of the laser beam is prevented.
The above-mentioned steps are carried out either in a condition in which the gas is sealed in the chamber or in a condition in which the gas is allowed to flow with the pressure being kept constant.
Then, the substrate is transferred from the chamber in which a laser beam is radiated to the substrate to a second chamber through which the substrate is transferred to another chamber. When the substrate is transferred in a vacuum condition, nitrogen or inert gas is stopped to be supplied into the chamber concurrently with stopping of introduction of oxygen gas and stopping of radiation of the laser beam, to thereby much exhaust the gas.
After it has been confirmed that the chambers are in almost the same pressure atmosphere, a gate valve separating the chambers is made open, and the substrate is transferred from the previous chamber to the second chamber.
When the substrate is transferred in oxygen atmosphere, the second chamber is caused to have a predetermined pressure in advance by inert gas, nitrogen gas or oxygen gas all having a high purity, alone or in combination. Then, the chamber in which the laser beam is radiated is caused to be in oxygen atmosphere, and the chamber is caused to have the same pressure as that of the second chamber. Thereafter, the gate valve separating the chambers from each other is made open, and the substrate is transferred to the second chamber.
Thus, it would be possible to prevent contaminants such as metal or carbon from adhering to a surface of a silicon film formed on a substrate, and prevent reduction in a transmission rate of laser beam through a laser-introducing window.